124-beta-invitations is a project mainly written in RUBY and JAVASCRIPT, it's free.
Railscasts Episode #124: Beta Invitations
http://railscasts.com/episodes/124
Commands
script/generate nifty_scaffold invitation sender_id:integer recipient_email:string token:string sent_at:datetime new script/generate migration add_invitation_to_users invitation_id:integer invitation_limit:integer script/generate mailer Mailer invitation